'Ön' is the polite form of 'you' that is conjugated in the third person singular, this is why '-sz' is missing from the ending. The casual form of 'You are walking' would be 'Te sétálsz.'
In my understanding; second person polite is third person, therefore you use the third person verb (no suffix) than the second person verb (-sz).
You wrote "You walks" instead of "You walk". Walks as a verb is only used with he/she/it in English, no matter what the Hungarian sentence says.
Also, the concept of "walk" and "are walking" are the same thing in Hungarian. Context usually tells the difference.